This proposal outlines the continuing development of toxin gene therapy for Acquired Immune Deficiency Syndrome (AIDS), based on regulated expression of a diphtheria toxin A (DT-A) gene. Plasmid an retroviral constructs were generated from which DT-A expression was controlled by HIV-1 regulatory proteins Tat and Rev. Stable cell lines were derived which had been transfected or infected with DT-A-encoding plasmids and retroviruses, respectively. Such cell lines showed a markedly impaired ability to produce HIV, especially those infected with DT-A retrovirus, which were resistant to infection by HIV, especially those infected with DT-A retrovirus, which were resistant to infection by HIV strain IIIb over at least a four day period. We now propose to examine the HIV resistance conferred by the DT-A retrovirus in more detail over a longer time course, and to improve the efficiency with which HIV-resistant cells are obtained. We will examine HIV resistance in systems selected to model, as closely as possible, in vivo HIV infection. We hope to demonstrate efficient inducibility of DT-A expression in HIV-infected cells, and cell death before substantial HIV production.